Networks and the Internet
What to know
Network questions test layered communication and addressing. Know the TCP/IP model (application, transport, internet, link) and how encapsulation adds headers at each layer. Compare TCP-connection-oriented, reliable, ordered, with flow and congestion control-to UDP-connectionless and low overhead. Learn IP addressing, subnet masks, CIDR, private ranges, default gateways, NAT, DHCP, DNS, ARP, routing versus switching, ports, HTTP/HTTPS, TLS, and firewalls. A switch forwards frames within a LAN by MAC address; a router forwards packets between networks by IP address. DNS resolves names to IP addresses; DHCP leases addresses; ARP maps IP to MAC on a local link.
Traps
Do not confuse MAC (link-layer, local) with IP (internet-layer, routable). HTTPS is HTTP over TLS, commonly port 443; HTTP is port 80; DNS uses 53. TCP is not always preferable-UDP suits live video and DNS queries. A /24 has 256 addresses but 254 usable host addresses because the network and broadcast addresses are reserved. Private ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
Worked example
Is 192.168.1.70 in 192.168.1.64/26? A /26 has 6 host bits, so 2^6 = 64 addresses. Blocks begin at .0, .64, .128, .192. The address .70 lies in .64–.127, so yes: network .64, broadcast .127, usable .65–.126. Rule of thumb: host bits = 32 − prefix; addresses per subnet = 2^host bits; usable IPv4 hosts = that minus 2.